1. Field of the Invention
This invention relates generally to well completion methods and apparatus, and in particular to improved methods and apparatus for perforating formations surrounding a well bore.
2. Description of the Prior Art
According to conventional well completion procedures, after a productive strata has been reached during the drilling of an oil or gas well, a well casing is run into the bore hole and is set in place by injecting a volume of cementitious material such as concrete into the annulus between the bore hole wall and casing wall. The annular layer of concrete anchors the well casing in place and seals the productive zone to prevent migration of formation fluids from one zone to another through the annular space. However, the annular volume of cement and the well casing block the flow of formation fluid into the interior of the casing. Therefore the concrete layer and the well casing must be perforated to permit the production of formation fluids to the surface.
In addition to perforating the well casing and cement material, it is sometimes desirable to perforate the surrounding formation in order to increase its permeability and enhance the flow of formation fluid to or from the formation. It is well known that many formations exist which contain large reserves of oil which cannot be recovered at a profitable rate due to the relatively low permeability of the formation or for other reasons. Attempts have been made to increase the production rate by fracturing the formation by the application of fluid pressure to develop cracks or fractures, and while such procedures have in many cases increased production, there are instances wherein the fracturing procedure has caused the loss of the well. One of the difficulties encountered with the formation fracturing methods is caused by the annular cement layer. According to conventional fracturing techniques, after a casing has been landed, and cement has been set between the bore hole wall and casing, the casing is punctured by a mechanical means such as a bullet or a shaped charge. Thereafter, a fracturing fluid is dicharged through the punctured casing to cause the formation to fracture. This also tends to rupture the bond between the cement and the bore hole wall. The fracturing fluid will take the path of least resistance and flow upwardly or downwardly along the cement interface rather than out into the formation.
When completing a well by shooting the casing with a bullet or a shaped charge, the bore hole will sometimes be considerably enlarged because of sloughing off of the formation, cave-ins and the like. When the casing is cemented in place, there exists a considerable lateral thickness of cement between the casing and formation which in some cases cannot be penetrated by conventional bullet or shaped charge explosives.
Because of the potential recovery from large reserves in a formation having a relatively low permeability, attempts have been made to increase the productivity of such wells by forming a channel from the bore hole laterally out into the formation. According to one technique, a whip stock is set in the casing and then after a mill has cut a window in the casing, a bit is run in on a flexible drill string and a hole is drilled out by rotating the bit into the formation. Although these procedures have been reasonably effective in forming the formation channels, the channels are usually so large as compared to the grain size of the formation sand to permit migration of the sand along with the formation fluids into the interior of the casing, thereby causing serious damage or destruction of equipment operating within the casing.
Although preperforated or slotted liners and screens can be run as part of the casing into the well bore and positioned adjacent the sand formation, it is sometimes desirable to be able to carry out completion operations at a different zone after the casing has been set. In such cases, it would be useful to be able to perforate the well casing adjacent a different production zone without removing it from the well bore.
Although quantum devices such as laser have been used for drilling holes in metal, their usage for perforating a well casing and the surrounding formation in situ has been believed to be impractical because of adverse conditions in the downhole environment. However, because the laser is capable of precise control for drilling relatively small channels through the well casing and surrounding formation without rupture, it seems to be otherwise well suited for well completion operations. There is, therefore, a continuing interest in adapting quantum devices such as lasers for use in downhole environments for carrying out well perforation and other completion operations.